Fuse structures



Nov. 18, 1958 K. w. SWAIN F USE STRUCTURES Filed Dec. 22, 1954 UnitedStates Patent FUSE STRUCTURES Kenneth W. Swain, Hampton Falls, N. H.,assignor to The Chase-Shawmut Company, N ewburyport, Mass.

Application December 22, 1954, Serial No. 476,936

15 Claims. (Cl. 200-131) This invention relates, generally, to electricfuses and to the protection of electric circuits by means of electricfuses and it has particular relation to current-limiting fuses and tothe protection of electric circuits having a low circuit voltage bymeans of such fuses.

It is one object of this invention to provide a currentlimiting fusewhich is easy to manufacture and which has considerable dimensionalstability.

Another object of this invention is to provide a currentlimiting fusehaving a ribbon-type fuse link, or a plurality of such links, which linkor links each have a portion of drastically reduced cross-sectional areato achieve a drastic limitation of fault currents, in which fuse thedanger of fracture of the link, or links, at the very fragile portion ofreduced cross-sectional area thereof is greatly reduced, or entirelyeliminated.

Another object of the invention is to provide a currentlimiting fusehaving a very steep time-current'characteristic or, in other words, acurrent-limiting fuse wherein the blowing time is very short in the lowovercurrent range.

Another object of the invention is to provide a very compactcurrent-limiting fuse particularly suitable for applications wherecurrent-carrying requirements and requirements as to current-limitingaction are high and the circuit voltage is relatively low, say less than100 volts.

Another object of the invention is to provide a currentlimiting fusecomprising terminal elements adapted to perform the dual function ofboosting the current-carryingcapacity of the fuse and of assisting inquenching the arc formed therein upon blowing thereof.

Another object of the invention is to provide a currentlimiting fusesuitable for application in the A.-C. and D.-C. circuits of dryrectifiers, such as germanium rectifiers.

The foregoing and other general and special objects of the invention andadvantages thereof will more clearly appear from the ensuing particulardescription of the invention, as illustrated in the accompanyingdrawings, wherein- Fig. 1 is a side elevation of a cartridge fuseembodying my invention;

Fig. 2 is in part a longitudinal horizontal section and in part a topplan view of the structure shown in Fig. 1;

Fig. 3 is a longitudinal vertical section of the structure shown inFigs. 1 and 2;

Fig. 4 is a transverse section along 44 of Fig. 2; and

Fig. 5 is a transverse section similar to that shown in Fig. 4 ofanother embodiment of the invention.

Referring now to the drawing, and more particularly to Figs. 1 to 4thereof, reference numeral 1 has been applied to indicate a pair ofspaced metallic terminal blocks, preferably made of copper. Terminalblocks 1, which are preferably cylindrical in shape, are arranged incoaxial relation to each other. Each of terminal blocks 1 is providedwith a blade contact 2 at the respective side thereof remote from thegap formed between, or the space bounded by, the two terminal blocks 1.Blade contacts 2 2,861,159 Patented Nov. 18, 1958 are provided withholes or perforations 2a to enable the fuse to be secured to the spacedcontacts of a fuse holder, or to a pair of spaced bus bars, etc., bymeans of screwthreaded studs (not shown).

In Fig. 2 the circuit with which the fuse is associated has beendiagrammatically indicated by two lines to which the reference letter Chas been applied. This circuit has a predetermined availableshort-circuit current. This term refers to the current which would flowin the circuit upon occurrence of a short-circuit formed by a conductorhaving practically zero impedance. The circuit of the fuse structureshown in Fig. 2 is a low-voltage circuit, in particular a circuit havinga circuit voltage of less than 100 volts, e. g. of 65 volts D.-C. Thefigure of 65 volts D.-C. is given by way of example and not intended tolimit the scope of the invention. The circuit of the fuse may be eithera D.-C. or an A.-C. circuit.

Each of the juxtaposed surfaces 10 of terminal blocks 1 is provided witha system of straight parallel grooves 7, 8. Grooves 7 extend across thecenters of the circular surfaces la. The additional grooves 8 on eachsurface 1a are arranged in spaced relation from, and on opposite sidesof, groove 7. A ribbon-type fuse link 5, preferably of silver, isinserted into groove 7 in blocks, and conductively connected toblocks 1. This may be achieved by joints of soft solder (not shown inthe drawing). Fuse link 5 has a neck or reduced cross-section portion 5asituated midway between blocks 1 and formed by a pair of lateral,substantially V-shaped notches, or incisions. The two converging edgesof the V-shaped notches defining neck 5a enclose an acute anglesubstantially less than deg. The cross-section of the portion of reducedcrosssectional area or neck 5a is less than A of the normalcross-sectional area of link 5, i. e. the cross-sectional area at anypoint of link 5 where it is laterally bounded by a pair of paralleledges. For electrical reasons neck So should be as narrow as possible,e. g. 34 of the width of link 5. As a result, link 5 is very fragile andmust be handled with great care before being assembled in a fusestructure. The length of the aforementioned portion of reducedcross-sectional area or neck 5a is preferably very short, i. e. so shortthat the portion of reduced crosssectional area or neck 5a approximatesa point heat source when link 5 is carrying an electric current. Thesmall width of the portion of reduced cross-sectional area or neck 5a isconducive to an intense current-limiting action or, in other words, to agreat reduction of let-through currents compared to the availableshort-circuit currents. The shortness of the portion of reducedcross-sectional area or neck 5a is conducive to small l .r losses in thefuse link, and therefore to a cool running fuse.

The two parallel spacing plates 6 inserted into grooves 8 maintain apredetermined spacing between terminal blocks 1 and thus establish thedimensional stability required for protection of the fragile necked link5 after the same has become an integral part of the fuse structure.Spacing plates 6 are made of a heat resistant electric insulatingmaterial comprising organic and inorganic constituents as, for instance,a synthetic-resin-glass-clothlaminate. A tin rivet 5b may be insertedinto link 5 if the fuse is intended to be used for protection againstprotracted overloads, in addition to protection against major faultcurrents. If the circuit voltage is in the order of 65 volts, link 5does not need to be immersed in a pulverulent arc-quenching filler. Atrelatively higher circuit voltages it is generally advisable to immersethe fuse link in a pulverulent arc-quenching filler, such as quartzsand. The casing 4 which consists of a relatively heat resistantinsulating material is mounted on the two cylindrical copper blocks 1and encloses the spacing plates 6 and the fuse link 5. A pair offerrules 3 of sheet metal may be mounted on the axially outer ends ofeasing 4. Casing 4 and terminal blocks 1 are attached to each other by aplurality of transverse steel pins 9.

The minimum length of a link of a current-limiting fuse is generallydeemed to be equal to the normal backburning distance of the link underthe most onerous interrupting conditions which may occur in the fusewhen interrupting a specified circuit. As a rule, the length of a fuselink is made considerably longer than the minimum length to provide asafety factor against action of the are upon the terminal elements ofthe fuse.

I have found that under certain conditions the length of the link orlinks of a current-limiting fuse may be considerably shorter than thenormal back-burning distance of the link or links under the most onerousinterrupting conditions which may occur in the fuse when the same isinterrupting a specified circuit, and that great advantages may beachieved by shortening a fuse link beyond the limits deemed heretoforeto be necessary and imperative. A fuse link may be made shorter than theaforementioned back-burning distance if the circuit voltage isrelatively moderate, particularly if the circuit voltage does not exceedthe order of 100 volts, and if the terminal elements of the fuse areformed by substantial metal blocks having such an intense cooling actionupon the are when contacted by the are as to cause instant extinction ofthe latter. In other words, the length of a link may be less than thenormally required minimum back-burning distance if the terminal elementsare capable of acting as are barriers and arc-quenchers, limitingfurther growth or elongation of the art and effecting instant quenchingthereof. Such conditions are present in the fuse structure and thecircuit shown in Figs. 1 to 4, inelusive.

The spacing S between the juxtaposed surfaces la of the cylindricalcopper blocks 1 is in the order of two or three times the width of theV-shaped, neck-forming notches at the open end thereof, and less thanthe normal back-burning distance of link 5 under the most severeinterrupting conditions which may occur in the fuse when interruptingthe circuit indicated by the reference letter C. As a result ofproviding the large copper masses 1 immediately adjacent thepoint-heat-source-forming neck 50 of fuse link 5, a novel kind of fuseperformance is achieved, i. e. the fuse is much faster in the lowoverload range than any other current-limiting fuse I am familiar with.This will become more apparent from what follows:

The performance of fuses may be described by their time-currentcharacteristic. showing the blowing time plotted against the currentcarried by the fuse. For fuses of the type shown in Figs. 1 to 3designed for shortcircuit protection only. thetime-current-characteristics are a family of straight lines if both theabscissae and the ordinates are plotted on a logarithmic scale. Eachline in this family of lines corresponds to one particular currentrating. All lines referring to a given type of fuses or design thereofhave the same slope.

It is of importance for certain applications as, for instance.protection of germanium rectifiers, to increase the slope of thetime-current-characteristic as much as possible. In other words. forcertain applications it is desired that the first derivative of thetime-current-characteristic be as high as po sible. The ideal forcertain applications would be to have a time-current-characteristicvirtually parallel to the axis of ordinates, i. e. parallel to the timeaxis. Such a fuse might be referred to as a ceiling device because ofits ability to carry a given ceiling current and to interrupt instantlyif that current is exceeded.

The fuse which has been shown in the patent drawing and has beendescribed above is the closest approach to a ceiling device I am awareof.

I have conducted tests with identical fuses, keeping all variablesunchanged, except the length of the link and the spacing between a pairof heavy terminal blocks of copper arranged at the axially outer ends ofthe link. It appears from these tests that the shorter the link and thesmaller the spacing between the heavy terminal blocks of copper, thelarger the current-carrying capacity of the link, and the larger thecurrent rating thereof. As a result of the up-rating of the fuse causedby reduction of the length of the link and of the distance between theheavy terminal blocks to less than the normal back-burning distance ofthe link under the severest interrupting conditions which the circuitmay impose upon the fuse, the fuse is much faster in the low overloadrange than any prior art fuse I am aware of. The low overload currentrange is deemed to be the range from about two times to about eighttimes the rated current of the fuse.

A fuse deemed to be very fast in the low overcurrent range is thatdisclosed in the copending patent application of Frederick J. Kozacka,Ser. No. 387,477, filed October 21, 1953, for Low-Voltage High-CapacityCurrent-Limiting Fuse, now United States Patent 2,734,111, issuedFebruary 7, 1956. The fuse disclosed in the copending Kozackaapplication has a blowing time in the order of hundredth of a second ifcarrying seven to eight times the rated current. A fuse built accordingto the teachings of the above copending patent application rated at 400amps. blew within 22 seconds When caused to carry 1200 amps, i. c. threetimes the rated current thereof. The identical links as those present inthis fuse were used in a number of tests in a circuit having a lowercircuit voltage, i. e. 65 volts, permitting to shorten the length of thelink and the spacing between the terminal blocks. Upon determination ofthe normal back-burning distance under the most severe interruptingconditions the circuit could impose upon the fuse, the length of thelink and the spacing between the heavy terminal blocks was reduced toless than said critical back-burning distance. With this reduced lengthof link and distance between terminal blocks the rated current of thefuse was 300 amps. When caused to carry three times its rated current,i. e. 900 amps. the fuse blew within 2.5 seconds. By virtue of the lowcircuit voltage and the effectiveness of the terminal blocks asarc-quenchers at such low voltage levels, the fuse interruptedconsistently in the absence of a pulverulent arc-quenching filler withinits casing. It will be apparent that the minimum distance between theterminal blocks depends upon the circuit voltage, and even if fulladvantage is taken of the arcquenching ability of the terminal blocks,relatively more severe interrupting conditions may require provision ofa pulverulent are quenching filler and some increase of the effectivelength of the link and of the spacing between the terminal blocks.Nevertheless, it is generally possible in circuits having a low circuitvoltage to achieve blowing of a fuse at about two to three times therated current thereof within five seconds, or at least less than tenseconds, provided that the spacing between the terminal blocks is lessthan the above referred-to critical back-burning distance, and providedthat full advantage is taken of the arc-quenching ability of theterminal blocks and a pulverulent arc-quenching filler added, ifnecessary.

It follows from the foregoing that where a low voltage are is elongatedby back-burning of a fuse link whose length is less than theback-burning distance required to extinguish the are under prevailingdeionizing conditions, the are may be instantly extinguished if blocksof cold metal are arranged in the path of potential growth of the are,which decreases the arc temperature instantly below the temperaturerequired for subsistence of the are and emission of electrons from thearc roots. To achieve the required cooling effect the masses of copperblocks 1 ought to be relatively large. It will be ob served from Figs. 2and 3 that the copper blocks 1 which are arranged within casing 4 occupya preponderant portion of the volume thereof.

The back-burning distance of a fuse link depends upon the design of thefuse of which the link forms an integral part, the circuit into whichthe fuse is inserted (circuit voltage, available current, frequency,power factor, number of phases) and upon the characteristics of thefault. The two most important characteristics of any fault are the faultstarting angle and the voltage angle at melting of the neck of the link.In a circuit which is highly inductive the available current will befully asymmetrical if the fault occurs at voltage zero. All otherconditions remaining unchanged, the maximum severity in a fuse Occurs ifarc initiation at the neck or restricted crosssection portion occursslightly before or at peak voltage. If the length of link 5 and thespacing between copper blocks 1 is less than the back-burning distanceof link 5 under the most severe interrupting conditions which may occurin the fuse when interrupting a specified circuit, and if the fuse iscalled to interrupt the circuit under less severe interruptingconditions than the circuit is capable of producing, only a portion ofthe link rather than the entire link may be consumed by the are formedincident upon blowing of the fuse.

Referring now to Fig. 5, this figure shows a cross-section of a fusestructure comprising a pair of spaced terminal elements of the same typeas those shown in Figs. 1 to 4, inclusive. The juxtaposed surfaces ofthe terminal elements are each provided with two parallel spacedlink-receiving grooves 7' and three parallel spaced grooves 8 forreceiving three insulating barriers or partitions 6'. The barriers orpartitions 6' operate as spacers for the terminal elements in the samefashion as the spacing plates 6 shown in Figs. 2 to 4, inclusive. Thebarriers or partitions 6' of the structure shown in Fig. 5 have theadditional function of precluding formation of one singlesemi-conducting fulgurite by the pulverulent siliceous arc-quenchingfiller within the easing 4'. It will be noted from Fig. 5 that threefuse links 5 are arranged to the left and three fuse links 5 to theright of the central barrier 6' of insulating material which thusprecludes the formation of a unitary fulgurite by the fuse links and thearc-quenching filler 10 arranged at different sides of central barrier6'. The two radially outer insulating barriers 6' are primarily used asspacers between the terminal elements of the fuse. Their fit in casing4' is a relatively loose one and metal vapors resulting from thevaporization of fuse link 5 are able to pass through the gaps formedbetween casing 4' and the radially outer barriers or spacing plates 6and to reach the body of pulverulent filler or quartz sand 10 situatedbeyond the radially outer plates 6'. Thus any portion of the body ofquartz sand 10 is effectively used to deenergize the products of arcing.

In both embodiments of the invention shown the linkrecetving groove orgrooves and the spacing-plate-receiving grooves may be cutsimultaneously by means of a suitable gang-type milling cutter. Thisreduces the manufacturing cost substantially compared to themanufacturing cost involved in manufacturing fuses comprislng spacingposts for the terminal elements rather than a plurality of spacedspacing plates. Structures of the kind shown in the patent drawingcomprising a plurality of spaced spacing plates extending between theterminal elements tend to have a higher degree of dimensional stabilitythan fuse structures having merely a spacing post between the terminalelements thereof.

The first step in assembling fuses of the type shown in the patentdrawing consists in making a dimensionally stable sub-assemblycomprising only the heavy terminal elements and their spacing plates ofinsulating material. This sub-assembly forms a self-contained structuralunit. The fuse link or links may thereafter be inserted into thelink-receiving grooves of this unit or sub-assembly without danger ofinjury to their very fragile neck or necks by a slight change in therelative position of the two terminal elements of the fuse structure.

The fact that the length of the link or links and the spacing betweenthe terminal elements is less than the normal back-burning distance ofthe link or links under the most severe interrupting conditions whichthe circuit is capable of imposing upon the fuse tends to keep the sizeof the fuse very small, requiring a minimum of bulk per amperecurrent-carrying capacity.

Whenever the term length of link or similar language is used in thiscontext, it is intended to refer to the effective length which is thelength situated within the gap formed between the juxtaposed surfaces 1aof terminal blocks 1 rather than to the actual length which includes theportion of link 5 beyond the surfaces In, i. e. the portion of the linkwithin groove 7. The actual length of a fuse link may be considerablylonger than its effective length.

Whenever the term rated current or similar language is used in thiscontext, it is intended to refer to a current which the fuse can carrycontinuously without undue heating. It appears from this definition thata current rating indicates a range of currents rather than a precisecurrent value. The above referred-to fuse which was rated at 300 amps.was actually able to carry currents of 350 amps. for an indefiniteperiod of time without undue heating. In other words, its rating of 300amps. is rather conservative. The minimum fusing current of this fusewas about 400 amps. Blowing thereof occurred at an overload of 1000amps, i. e. 2.5 times the minimum fusing current, in .2 second which isagain much less than any prior art current-limiting fuse I am familiarwith. Speaking in terms of rated current rather than minimum fusingcurrent, the fuse blew in .2 second at 3.33 to 2.87 times the ratedcurrent, and the rated current was .3 to .35 times the current requiredto cause blowing of the fuse within .2 second.

As mentioned above it is often desirable to make the neck or reducedcross-section portion very narrow. A fuse link having a neck width of ofthe normal link width imposes less difficult problems than a linkwherein the ratio of neck width to normal link width is considerablysmaller. With decrease of that ratio the links become increasinglyfragile, and decrease of that ratio may also tend to cause transientovervoltages incident to blowing of the fuse since it may result inincreased rates of change of current.

Where utmost dimensional stability on account of particularly small neckdimensions is indicated, it is desirable to apply a high local pressureto opposite sides of blocks 1 at right angles to the planes of spacingplates 6 in order to drive or displace some of the copper of whichblocks 1 are made radially inwardly into grooves 8, thereby reducing thecross-sectional area thereof. This locks spacing plates 6 firmly intotheir grooves 8. Such a sub-assembly has sufficient dimensicnalstability to safely receive the most fragile ribbon-type fuse link.

I have found that in current-limiting fuses having fuse links with necksclosely approximating a point heat source the current-limiting actionmay be so drastic as to require a means for decelerating current decayfrom the limited peak of the let-through current down to zero. Omissionof an aroquenching filler within the casing of the fuse combined with areduction of the size of the arcing chamber have been found to be thesimplest means to control the rate if current-decay in such a way as topreclude dangerous voltage surges. By assigning the preponderant portionof the volume of the casing 4 to the terminal blocks 1, and omitting anarc extinguishing filler within the small arcing and deionizing chamberleft within the casing 4 of the fuse, the rate of current decay may bekept within the relatively narrow limits imposed by very criticalapplications such as, for instance, protection of dry rectifiers, e. g.germanium rectifiers.

It will be understood that I have illustrated and described herein twopreferred embodiments of my invention and that various alterations maybe made in the details thereof without departing from the spirit andscope of my invention as defined in the appended claims.

I claim:

1. A fuse comprising a pair of spaced metallic cylindrical blocks, aplurality of substantially spaced grooves in each of the juxtaposedsurfaces of said pair of blocks, a plurality of spacing plates of anelectric insulating material inserted into said plurality of grooves tomaintain a predetermined spacing between said pair of blocks. fuse linkmeans spaced from said plurality of spacing plates conductivelyinterconnecting said pair of blocks, and a casing of insulating materialmounted on said pair of blocks enclosing said plurality of spacingplates and said fuse link means.

2. A fuse comprising a pair of spaced cylindrical metal blocks, threespaced parallel grooves in each of the juxtaposed surfaces of said pairof blocks, a pair of parallel spacing plates of an electric insulatingmaterial inserted into the outer of said grooves to maintain apredetermined spacing between said pair of blocks, a ribbon-type fuselink having a portion of restricted cross-sectional area situatedbetween the axially outer ends thereof inserted into the inner of saidgrooves and conductively connected to each of said pair of blocks, and acasing of insulating material mounted on said pair of blocks enclosingsaid pair of spacing plates and said fuse link.

3. A fuse comprising a pair of spaced coaxially arranged substantiallycylindrical metal blocks each provided with a blade contact projectingfrom the axially outer surface thereof, three spaced grooves in each ofthe juxtaposed surfaces of said pair of blocks, a pair of parallelspacing plates of a heat resistant electric insulating material havingorganic and inorganic constituents inserted into the outer of saidgrooves to maintain a pre determined spacing between said pair ofblocks, a ribbontype fuse link having a portion of restrictedcross-sectional area less than of the normal cross-sectional of saidlink inserted into the inner of said grooves to conductivelyinterconnect said pair of blocks, and a tubular casing of insulatingmaterial mounted on said pair of blocks enclosing said pair of spacingplates and said fuse link.

4. A fuse comprising a pair of spaced coaxially arranged substantiallycylindrical metal blocks each provided with a blade contact projectingfrom the axially outer surface thereof, a first groove in each of thejuxtaposed surfaces of said pair of blocks extending across the centerthereof, a pair of additional grooves in each of said juxtaposedsurfaces, each of said pair of additional grooves being arranged inrelation on opposite sides of and parallel to said first groove, a pairof spacing plates of a heat resistant synthetic-resinglasscloth-laminateinserted into said pair of additional grooves in said pair of blocks tomaintain a predetermined spacing thereof, a ribbon-type fuse link havinga portion of restricted cross-sectional area less than A; of the normalcross-sectional area thereof inserted into said first groove in eachsaid pair of blocks to conductively interconnect said pair of blocks,and a tubular casing mounted on said pair of blocks enclosing saicl pairof spacing plates and said fuse link.

5. In combination an electric circuit having a circuit voltage less than100 volts and a short-circuit current, a current-limiting fuse arrangedin said circuit for limiting major fault currents therein to values lessthan said available short-circuit current, said fuse comprising a fuselink having a portion of restricted cross-sectional area less than 1 ofthe normal cross-sectional area of said link, and a pair of spaced metalblocks arranged at the axially outer ends of and conductively connectedwith said link. the spacing between juxtaposed surfaces of said metalblocks being less than the normal back-burning distance of said linkunder the most onerous interrupting conditions which may occur in saidfuse when interrupting said circuit.

6. In combination an electric circuit having a circuit voltage of lessthan volts and a predetermined available short-circuit current, acurrent-limiting fuse arranged in said circuit for limiting the majorfault currents therein to values less than said available short-circuitcurrent, said fuse comprising a fuse link having a short portion ofrestricted cross-sectional area approximately a point heat source whensaid link is carrying an electric current, a pair of spaced coppercylinders coaxially arranged at the axially outer ends of andconductively connected to said link, grooves in juxtaposed surfaces ofsaid copper cylinders, spacing plates of insulating material insertedinto said grooves to maintain a predetermined spacing between saidjuxtaposed surfaces, said predetermined spacing being less than thenormal back-burning distance of said link under the most onerousinterrupting conditions which may occur in said fuse when interruptingsaid circuit.

7. In combination a circuit having a circuit voltage of less than 100volts and a predetermined available shortcircuit current, acurrent-limiting fuse arranged in said circuit for limiting major faultcurrents therein to values less than said available short-circuitcurrent, said fuse comprising a fuse link having a short portion ofrestricted cross-sectional area approximating a point heat source whensaid link is carrying an electric current, a pair of spaced metalcylinders coaxially arranged at the axially outer ends of andconductively connected to said link, a first pair of grooves each in oneof the juxtaposed surfaces of said pair of metal cylinders and eachreceiving one of said axially outer ends of said link, a pair ofadditional grooves in each of said juxtaposed surfaces, each of saidpair of additional grooves being arranged in spaced relation on oppositesides of and parallel to one of said first pair of grooves, a pair ofparallel spacing plates inserted into said pair of additional grooves insaid juxtaposed surfaces to maintain a predetermined spacingtherebetween, said predetermined spacing being less than the normalback-burning distance of said link under the most severe interruptingconditions which may occur in said fuse when interrupting said circuit.

8. In combination an electric circuit having a circuit voltage of lessthan 100 volts and a predetermined available short-circuit current, acurrent-limiting fuse arranged in said circuit for limiting major faultcurrents therein to values less than said available short-circuitcurrent, said fuse comprising a ribbon-type fuse link having anextremely short portion of restricted cross-sectional area approximatinga point heat source when said link is carrying an electric current, apair of spaced metal blocks arranged at the axially outer ends of andconductively interconnected by said link, the length ofsaid link and thespacing between said pair of metal blocks being less than the normalback-burning distance of said link under the most severe interruptingconditions which may occur in said fuse when interrupting said circuit.

9. in combination an electric circuit having a predetermined availableshort-circuit current, a current-limiting fuse arranged in said circuitfor limiting major fault currents therein to values less than saidavailable shortcircuit current, said fuse comprising a fuse link havinga portion of restricted cross-sectional area less than of the normalcross-sectional area of said link, a pair of spaced metallic terminalblocks arranged at the axially outer ends of said link sufficientlyclose to said portion of restricted cross-sectional area to impart acurrent-rating to said fuse in the order of .3 times the currentrequired to cause blowing of said fuse in the order of .2 second, and apair of spacing plates of insulating material arranged on opposite sidesof and substantially spaced from said link to maintain the spacingbetween said pair of terminal blocks.

10. In combination an electric circuit having a circuit voltage of lessthan 100 volts and a predetermined shortcircuit current, acurrent-limiting fuse arranged in said circuit for limiting major fauftcurrents therein to values less than said available short-circuitcurrent, said fuse comprising a fuse link having a portion of restrictedcross-sectional area less than 4 of the cross-sectional area of saidlink, a pair of spaced metallic terminal blocks arranged at the axiallyouter ends of said link sufiiciently close to said portion of restrictedcross-sectional area to impart a current rating to said fuse in theorder of .3 times the current required to cause blowing of said fusewithin about .2 second, and a pair of spacing plates of a heat resistantsynthetic-resin-glass-cloth-laminate arranged on opposite sides of saidlink in substantially spaced relation thereto to maintain the spacingbetween said pair of terminal blocks.

11. In combination an electric circuit having a circuit voltage of lessthan 100 volts and a predetermined available short-circuit current, acurrent-limiting fuse arranged in said circuit for limiting major faultcurrent therein to values less than said available short-circuitcurrent, said fuse comprising a casing of insulating material, a pair ofspaced metal terminal blocks arranged on opposite ends of said casingand occupying a preponderant portion of the volume thereof, a ribbontype fuse link having a portion of restricted cross-sectional areaadapted to approximate a point heat source when said link is carrying anelectric current conductively interconnecting said pair of metal blocks,said pair of metal blocks being arranged sufliciently close to saidportion of restricted cross-sectional area of said link to impart acurrent rating to said fuse of more than .3 times the current requiredto cause blowing of said fuse in about .2 second, and a pair of spacingplates of insulating material each arranged on opposite sides of saidlink to maintain the spacing between said pair of metal blocks.

12. In combination an electric circuit having a predetermined availableshort-circuit current, a current-limiting fuse arranged in said circuitfor limiting major fault currents therein to values less than saidavailable short-circuit current, said fuse comprising a fuse link havinga portion of restricted cross-sectional area less than the normalcross-sectional area of said link and sufliciently short to approximatea point heat source when said link is carrying an electric current, apair of metallic terminal blocks conductively connected by said linkequally spaced from said portion of restricted cross-sectional area andspaced at distance from each other less than the back-burning distanceof aid link under the most onerous interrupting conditions which mayoccur in said fuse When interrupting said circuit, and a pair of spacersof insulating material each arranged to one side of said link forrigidly maintaining said distance between said pair of terminal blocks.

13. In combination an electric circuit having a circuit voltage of lessthan 100 volts and a predetermined available short-circuit current, acurrent-limiting fuse arranged in said circuit to limit majorfault-currents therein to values less than said available short-circuitcurrent, said fuse comprising a substantially tubular casing ininsulating material, a pair of spaced terminal blocks of metal arrangedwithin at opposite ends of said casing and occupying a preponderantportion of the volume thereof, a ribbon-type fuse link having a portionof restricted crosssectional area less than $5 of the normalcross-sectional area of said link conductively interconnecting said pairof blocks, the arcing chamber defined by said casing and the juxtaposedsurfaces of said pair of blocks being solely filled with atmospheric airas an arc-extinguishing medium, and the spacing between said juxtaposedsurfaces being less than the normal back-burning distance of said linkunder the most severe interrupting conditions which may occur in saidfuse when interrupting said circuit.

14. A current-limiting fuse comprising a pair of spaced metallicterminal elements, a plurality of grooves in each of the juxtaposedsurfaces of said pair of terminal elements, a plurality of spacingplates of an electric insulating material inserted into said pluralityof grooves to maintain a predetermined spacing between said pair ofterminal elements, means for locking said plurality of spacing plates insaid plurality of grooves including portions of the metal of which saidpair of terminal elements is made displaced into said plurality ofgrooves to reduce the cross-sectional area thereof, a ribbon-type fuselink conductively interconnecting said pair of terminal elements, and acasing enclosing said plurality of spacing plates and said fuse link.

15. In combination an electric circuit having a predetermined availableshortcircuit current at a given point thereof, a current-limiting fusearranged in said circuit at said given point for limiting fault currentstherein to values less than said available short-circuit current, saidfuse comprising a ribbon-type fuse link having a pair of substantiallyV-shaped lateral notches, said pair of notches jointly defining a shortportion of restricted crosssectional area between the closed endsthereof approximating a point heat source when said link is carrying anelectric current, a pair of spaced coaxial cylindrical metal blocksarranged at the axially outer end of and conductively interconnected bysaid link, the length of said link and the spacing between said pair ofmetal blocks being sufficiently small to impart a current rating to saidfuse in the order of .3 times the current required to cause blowing ofsaid fuse in about .2 second, and a substantially tubular casing housingsaid link mounted on said pair of metal blocks.

References Cited in the file of this patent UNITED STATES PATENTS652,748 Cote July 3, 1900 1,388,269 Kramer Aug. 23, 1921 1,953,011 CoteMar. 27, 1934 2,665,348 Kozacka Jan. 5, 1954 2,670,418 Kozacka Feb. 23,1954 2,734,111 Kozacka Feb. 7, 1956 2,734,112 Kozacka Feb, 7, 1956UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No.2,861,150

November 18, 1958 Kenneth W. Swain pecification n requiring correctionand that the said Letters Patent should read as corrected below.

Column 2, line 50, for "l .r" read I .r column 8, line 9,

for "approximately" read approximating Signed and sealed this 3rd day ofMarch 1959.

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner ofPatents

